The present invention relates to single-walled nanotubes and methods for producing the same. In particular, the present invention relates to single-walled nanotubes having filled or partially-filled lumens and to methods for producing the same.
Since their discovery, single-walled carbon nanotubes (SWCNTs) have stimulated widespread scientific research due to their promising electronic and mechanical properties. Proven techniques for the production of SWCNTs include carbon arc-discharge evaporation (Iijima et al., xe2x80x9cSingle-shell carbon nanotubes of 1-nm diameterxe2x80x9d, NATURE, Vol. 363, pp. 603-604, Jun. 17, 1993; Ajayan et al., xe2x80x9cGrowth morphologies during cobalt-catalyzed single-shell carbon nanotube synthesisxe2x80x9d, Chem Phys. Lett., 215(5), pp. 509-517, Dec. 10, 1993; Bethune et al., xe2x80x9cCobalt-catalysed growth of carbon nanotubes with single-atomic-layer wallsxe2x80x9d, NATURE, vol. 363, pp. 605-606, Jun. 17, 1993), pulsed laser vaporization (PLV) of graphite in the presence of certain metallic catalysts (Thess et al., xe2x80x9cCrystalline Ropes of Metallic Carbon Nanotubesxe2x80x9d, SCIENCE, vol. 273, pp. 483-487, Jul. 26, 1996), and chemical vapor deposition.
The known methods for producing SWCNTs, however, are unable to produce high quality SWCNT material. Accordingly, the material is refluxed in nitric acid to destroy contaminants that are more easily oxidized than the SWCNTs and then filtered to yield a sheet of tangled nanotubes a fraction of a millimeter thick (i.e., bucky paper). The bucky paper is then vacuum annealed at 1100xc2x0 C. to drive off C60 molecules and other residual organic impurities.
SWCNTs, produced using a pulsed laser vaporization (PLV) process similar to the one described above, have been investigated using high resolution transmission electron microscopy (HRTEM). Since a carbon nanotube satisfies the weak phase object approximation, its image is a projection of the specimen potential onto a plane that lies normal to the electron beam. The image has maximum contrast where the beam encounters the most carbon atoms, which occurs where the electron beam is tangent to the graphene walls of the carbon nanotube. Indeed, when SWCNTs which are pulled away from the bulk bucky paper were examined using HRTEM, an image consisting of two dark parallel lines separated by about 1.4 nm was obtained. The fact that the space between the parallel lines was clear indicates that no material is present within the SWCNTs. Characterizations of purified PLV-produced nanotubes by X-ray diffraction and Raman spectroscopy confirm these results.
Presently, a SWCNT has the largest elastic modulus (i.e., stiffness) of any intrinsic material. Accordingly, SWCNTs have been used to form reinforced polymer-matrix composites which are useful as high strength, light weight materials. In general, the mechanical properties of the reinforced composites are improved by increasing the stiffness of the reinforcing fibers. As a result, there is a continued need for carbon fibers having even larger elastic modulii.
It has been shown that the stiffness of SWCNTs can be increased by filling it with certain small molecules. Like any other material, when a SWCNT is placed in tension, it undergoes a Poisson contraction that causes a reduction in diameter. By filling SWCNTs with a molecule having a small compressibility, such as C60, the Poisson contraction is resisted and the elastic modulus is increased. This resistance to transverse deformation also indicates that SWCNTs containing C60 should be less likely to debond from the surrounding matrix, minimizing the likelihood of fiber pull-out, a common failure mode in composites. Use of such filled SWCNTs as reinforcing fibers in polymer-matrix composites should therefore provide a means for improving upon the mechanical properties of polymer-matrix composites.
It has been shown that exterior adsorbates can change metallic SWCNTs into semiconducting SWCNTs, opening the possibility for all-carbon metal-semiconductor junctions at the nanometer length scale. Dopants intercalated in between nanotubes have also been shown to affect the tubes""electronic properties. However, the usefulness of exterior molecules to modify the intrinsic properties of nanotubes is limited by the fact that exterior molecules are accessible to chemical reaction and may be unstable in solvents, vacuum, or certain atmospheres. Interior molecules are hermetically sealed within the nanotube cores, with the nanotube itself forming a steric and kinetic barrier to reaction. It is expected that certain interior molecules, like exterior molecules, will modify the electronic properties of nanotubes. Therefore, it would be highly beneficial to provide a means for permanently modifying the electronic properties of nanotubes by filling to produce novel devices, interconnects, and other technologies.
Mass transport inside a nanotube has been demonstrated by the motion of encapsulated C60 along the axis of the surrounding tube. By the encapsulation of molecules having a strong magnetic moment, one-dimensional, nanoscopic mass transport devices which can be driven by an applied magnetic field are possible. Since even strong magnetic fields are known to be bio-compatible, this has potential application in the targeted delivery of drug molecules by the field induced ejection of the molecules from the nanotube cavity into the target (i.e., a nano-syringe). Mass transport may also be driven by other means (e.g., by momentum transfer from a laser). In all cases, the nanotube can be utilized to direct the motion of molecules in a controlled manner and along specific pathways.
Nanotubes also show promise for use as chambers to produce or catalyze the production of molecular structures that could not be easily produced otherwise. For example, a nanotube can act to enhance a reaction rate by confining reactants in close proximity, and it can determine the reaction product by providing a steric constraint on its structural form. Despite its small diameter, a nanotube is sufficiently strong to be self supporting over distances that are large as compared to many molecules. The nanotube therefore provides a convenient means to stabilize individual molecules for high signal-to-noise ratio characterization, such as by electron beam techniques, without the interference from a substrate film.
In light of the foregoing, SWCNTs and other types of single-walled nanotubes (SWNTs) show promise as reinforcing fibers for polymer-matrix composites, novel electronic devices, and nanoscopic mass transport devices (including nanoscopic drug delivery systems). The use of nanotubes in such applications and others is dependent upon the ability to controllably modify the intrinsic (e.g., mechanical, electronic, and/or magnetic) properties of the SWNTs by manipulating their microstructure. A particularly promising means for altering the intrinsic properties of SWNTs and using SWNTs in novel ways involves the filling of nanotube cavities or lumens to produce hybrid molecular assemblies that can have novel functionality. Accordingly, there is a need for nanotubes containing small molecules within their interior lumens and methods for producing the same.
The present invention relates to single-walled nanotubes having filled or partially filled lumens and methods for producing the same. The single-walled nanotubes are filled or partially filled with molecular species that are expected to alter the microstructure of the single-walled nanotubes. As such, the intrinsic (e.g., mechanical, electronic, and/or magnetic) properties of the filled or partially filled nanotubes can be controllably modified making the filled or partially filled nanotubes useful as reinforcing fibers for polymer-matrix composites, novel electronic devices, and nanoscopic mass transport devices.
In one of its aspects, the present invention relates to a hybrid material comprising a single-walled nanotube and a fill molecule contained within a lumen of the nanotube.
In another of its aspects, the present invention relates to a bulk material comprising nanotubes. The nanotubes are characterized by the fact that at least about 5%, preferably at least about 50%, and more preferably at least about 80% of the nanotubes contain one or more fill molecules within their lumens. In one embodiment, the nanotubes are arranged as a sheet of nanotubes.
In yet another of its aspects, the present invention relates to a method for producing a hybrid material comprising a single-walled nanotube having a lumen and a fill molecule contained within the lumen of the single-walled nanotube. The method comprises the step of forming a precursor material. The precursor material comprises nanotubes produced using any of a variety of techniques, including carbon arc-discharge evaporation, chemical vapor deposition, and pulsed laser vaporization. The nanotubes are optionally purified or similarly treated. The precursor material is then contacted with a fill molecule to bring the precursor material in physical contact with, or close proximity to, the fill molecules. In one embodiment, the precursor material is contacted with the fill molecule under conditions of temperature, pressure, and time sufficient to allow one or more fill molecules to at least partially fill the lumens of the nanotubes. The precursor material is then optionally heat treated to further induce the fill molecules to enter the lumens of the SWNTs. In one embodiment, the precursor material is heat treated by annealing the precursor material at a temperature below about 1000xc2x0 C., preferably below about 800xc2x0 C., and most preferably below about 600xc2x0 C.
Additional features and embodiments of the present invention will become apparent to those skilled in the art in view of the ensuing disclosure and appended claims.
The present invention relates to a hybrid material comprising a single-walled nanotube (SWNT) having a lumen and a fill molecule contained within the lumen of the SWNT.
The SWNT is formed using any of a variety of techniques known in the art. For example, the SWNT can be formed using pulsed laser vaporization (PLV) as described, for example, in Rinzler et al., Appl. Phys. A 67, pp. 29-37 (1998), the disclosure of which is incorporated herein, in its entirety, by reference. Alternatively, the SWNT can be synthesized using chemical vapor deposition. In still another embodiment, the SWNT is formed using carbon arc-discharge evaporation as described, for example, in Iijima et al., xe2x80x9cSingle-shell carbon nanotubes of 1-nm diameterxe2x80x9d, NATURE, Vol. 363, pp. 603-604, (1993); Ajayan et al., xe2x80x9cGrowth morphologies during cobalt-catalyzed single-shell carbon nanotube synthesisxe2x80x9d,Chem Phys. Lett., 215(5), pp. 509-517, (1993); and Bethune et al., xe2x80x9cCobalt-catalysed growth of carbon nanotubes with single-atomic-layer wallsxe2x80x9d, NATURE, vol. 363, pp. 605-606, (1993), the disclosures of which are all incorporated herein, in their entireties, by reference. Any of a variety of single-walled nanotubes are contemplated for use with the present invention, including without limitation single-walled carbon nanotubes and boron-nitride nanotubes.
The fill molecule that is contained within the SWNT can take a variety of forms. In one embodiment, the fill molecule comprises one or more C60 molecules. Since C60 molecules have a diameter of about 0.7 nm and a graphitic van Der Waals spacing of about 0.3 nm, the C60 molecules fit within the lumen of a SWNT having a diameter of about 1.4 nm without significant distortion from its equilibrium configuration. When the SWNT contains more than one C60, the structure resembles that of a peapod, where the SWNT is the xe2x80x9cpodxe2x80x9d and the C60 molecules are the individual xe2x80x9cpeas.xe2x80x9d
In another embodiment, the fill molecule comprises one or more capsule shaped fullerenes in the form of a cylinder of carbon, preferably having 10n carbon atoms, wherein nxe2x89xa70. Each capsule is optionally capped at one end, both ends, or neither end by hemispherical carbon xe2x80x9ccapsxe2x80x9d. The carbon cap is essentially one half of a C60 molecule. Accordingly, when the capsule is capped on both ends, the fullerene contains 60+10n carbon atoms. Further, when the capsule is capped on both ends, the capsule can be described as a metastable form of a fullerene. It should be apparent that as n increases, the capsule approximates a single-walled carbon nanotube, such that the hybrid material approaches a co-axial tube (CAT) structure.
In still another embodiment, the fill molecule comprises one or more metallofullerenes. Metallofullernes are fullerenes containing any of a variety of chemical species, including one or more transition metal elements. For example, each fill molecule comprises two lanthinum atoms surrounded by, or endohedrally contained within, a C80 cage (La2@C80).
The hybrid materials can be formed using a method in accordance with the present invention wherein the hybrid material is formed from a precursor material comprising one or more SWNTs. The SWNT is formed using any of a variety of techniques known in the art. For example, the SWNT can be formed using pulsed laser vaporization (PLV) , chemical vapor deposition, or carbon arc-discharge evaporation.
Once the SWNT has been formed, the SWNT is optionally purified or similarly treated. For example, an acid (e.g., nitric acid) treatment may be imposed to remove contaminants that are more easily oxidized than the SWNTs. The SWNTs are then optionally filtered to recover the solid.
The precursor material is then contacted with the fill molecules to bring the precursor material in physical contact with, or close proximity to, the fill molecules. In one embodiment, the precursor material is contacted with the fill molecules under conditions of temperature, pressure and time sufficient to allow one or more fill molecules to at least partially fill the lumens of the SWNTs. In another particular embodiment, the fill molecules are themselves by-products of the process used to form the SWNT. For example, when the SWNT is formed by PLV and the resulting material is acid treated, the fill molecules may appear as a residue on the outer surfaces of the SWNT.
Alternatively, solution and/or vapor based transport can be used to contact the SWNTs with the fill molecules. In the former case, a solvent (e.g., dimethyl formamide or toluene) is chosen to permit the molecules to wash over and come in contact with the nanotubes. In the latter case, the temperature and/or pressure is tuned to permit the molecules to enter the vapor phase. These steps may be performed under vacuum, in an inert or non-oxidizing environment, or in air. The mobile molecules then collide with the nanotubes and enter through openings in their walls, possibly assisted by diffusion along the nanotubes"" exterior and interior surfaces.
The precursor material is then optionally heat treated to further induce the fill molecules to at least partially fill the lumens of the SWNTs. In one embodiment, the precursor material is annealed at a low temperature to produce the hybrid material. A minimum temperature of about 300xc2x0 C., preferably about 325xc2x0 C., and more preferably about 350xc2x0 C. is achieved at a vacuum of about 3xc3x9710xe2x88x925 Pa to promote exterior fill molecules (e.g., C60) to enter the tubes on a reasonable time scale. Additionally, the precursor material is annealed at a temperature below about 1000xc2x0 C., preferably below about 800xc2x0 C., and more preferably below about 600xc2x0 C. Higher annealing temperatures limit the residence time of C60 on a SWNT as well as heal the nanotubes"" walls, thereby eliminating access to their interiors. Since formation is governed by both time and temperature, nanotube material is preferably soaked in, and not ramped through, this temperature window in order to produce filled SWNTs in abundance. The precursor material is annealed for a time sufficient to effectuate the production of the fill molecule within the lumen of the SWNT. For example, anneal times from about 1 hour to about 100 hours, and preferably from about 1 hour to about 24 hours, can be utilized.
The filled or partially filled SWNTs are optionally further treated to induce reactions between fill molecules within the lumens of the SWNTs. For example, when the SWNTs are filled or partially filled with fullerenes (e.g., C60), heat treating the SWNTs at a temperature above about 1000xc2x0 C., preferably above about 1100xc2x0, and more preferably above about 1200xc2x0 C. causes the individual fullernes to coalesce to form extended fullerenes (e.g., capsules).